The in situ production of continuous, jointless concrete pipe in open trenches is now widely accepted. It yields high-quality pipe at modest costs. Under one method pipe is produced by employing the bottom of an open trench as the supporting medium for the lower pipe half while forms are provided to support the upper pipe half until the concrete has hardened. The forms are relatively short lengths of metal sheets having a curved shape complementary to the interior curvature of the pipe. After the concrete has hardened, the forms are removed by workmen who enter the pipe, release the forms and then remove them by dragging them to the open end of the pipe. Methods and apparatus for constructing such pipe are described, for example, in U.S. Pat. Nos. 2,731,698; 3,106,760; 3,113,364; and 3,534,449. U.S. Pat. No. 3,785,759 proposes to delete interior pipe forms for supporting the upper pipe half while the concrete hardens. As far as is known to applicant, the method proposed in the last mentioned patent is technically and/or economically not feasable and has not been employed on a commercial scale.
Although the method and machines proposed in the above patents have resulted in the development of an effective technique for making and installing large diameter concrete pipe in a single operation they are subject to disadvantages and shortcomings. For example, with increasing pipe diameters the slump of the concrete mix must be decreased to prevent a collapse of the freshly poured concrete pipe before the concrete has hardened. Such an increasingly dry mix, however, results in reduced flow rates of the concrete which, in turn, can lead to the formation of voids. With prior art machines this is prevented by vibrating the freshly poured concrete with electrically actuated vibrators which are either manually operated or attached to sides of the pipe laying machine. In an open trench this practice has drawbacks since it can loosen the surrounding soil, particularly if it is sandy soil. Once the soil is loosened it flows to the bottom of the trench where it fills spaces that should be filled with concrete, forming discontinuities in the concrete which reduce effective wall thicknesses of the pipe that can lead to premature pipe failures. Moreover, the practice of vibrating the concrete with vibrators attached to the machine, as is done in prior art machines, subjects the whole machine, including the molding sections thereof together with the forms and spreaders, to vibrations which are transmitted to the freshly poured concrete pipe, sometimes resulting in movement of the concrete and the formation of cracks in the crown of the pipe before the concrete is fully set.
A further shortcoming of prior art continuous concrete pipe laying machines was the fact that the trench had to be made oversize to enable the movement of the machine through the trench. The forward end of the machine, frequently referred to as the "sled", was adequately guided along and centered by the vertical sides of the trench. However, the aft portion of the machine, which includes the concrete forming molds, that is the interior mandrel including an interior, lower concrete trowelling plate and the exterior mold, or the upper trowelling plate, which shapes the upper exterior surface of the pipe, was largely unsupported by the side walls. Consequently, the aft portion of the machine, and therewith, the molds could move laterally relative to the trench sides. This is turn resulted in corresponding, highly undesirable variations in the wall thickness. To assure a minimum design wall thickness, the average wall thickness, therefore, had to be larger than the minimum design wall thickness. This resulted in a substantially larger concrete consumption for laying the pipe than would be necessary if such lateral mold movements did not take place.
Another shortcoming of the prior art relates to the problem of removing the forms and spreaders from the freshly formed pipe after the concrete sets and hardens. Presently, this requires considerable time and effort which renders the operation costly. The present invention modifies the spreaders to render this operation easier and faster to perform.
An additional and costly shortcoming of the prior art concrete pipe laying machine is the fact that portions of the machine, particularly the concrete receiving hopper, protrude well above the top exterior surface or crown of the pipe being poured. This causes problems whenever the established grade of the pipe line requires the pipe to be laid beneath crossing utility lines and the like unless the clearance between the obstacle and the crown of the pipe is at least about 12 inches, the approximate height that the machine high point above the pipe crown.
Since general engineering practice does not allow even temporary deviations of more than 4 inches from the established grade, whenever the clearance below the obstacle, utility line of the like is greater than 4 inches, it is necessary to discontinue the concrete pouring process, lift the machine from the trench, replace it in the trench on the other side of the obstacle, and then resume the concrete pouring process. The intermediate section of the pipe line between the two poured sections has to be completed with pre-cast concrete pipe or the like.
This is obviously a time consuming and costly operation. Depending on the size of the pipe being poured, such costs can run as hgih as or higher than $1,000 per discontinuity.